1. Field of the Invention
The field of the invention is electrical audio-signal processing, systems, and devices. The Active Sound Absorber of the invention is based on an electroacoustical transceiver, defined as a bilateral electro-acoustical transducer acting as both a diaphragm actuator and motion sensor, and an associated mutual inductance discriminator in a electroacoustical feedback system. The selected embodiment of such a system is an unvented hearing aid where the Active Sound Absorber combats the occlusion effect.
2. Description of the Related Art
Sound absorbers based on acoustical resonance have been used since they were originally proposed by Helmholtz about a century ego. Sound absorbers are used in a variety of products from perforated ceiling tiles to the so called "silators" proposed by Oskar Bschorr in U.S. Pat. Nos. 4,149,612, 4,325,458 and 4,325,461. Passive resonant sound absorbers of the art are effective only in a limited-frequency range close to the resonance frequency of the device. At resonance, the mass and compliance components of a series acoustical resonance network of the absorber cancel, and the absorber acts as a low-impedance acoustical resistor, absorbing the impinging sound. Low-frequency absorbers are inconveniently large.
Addition of an external sensor and actuator to drive a passive silator was proposed by Bschorr in a technical paper: "An Integrated Microphone/Loudspeaker Unit for Active Noise Cancellation" given at Inter-Noise meeting in Cambridge, Mass., Jul. 21-23, 1986. (The Proceedings of this meeting have been edited by R. Lotz and published by the Noise Control Foundation). Such a three element active-sound absorber design is complex and bulky. Further external components each add delay and phase shift which complicates the feedback loop stabilization and limits the useful frequency range of such an active absorber. The design of the appropriate feedback circuit was not addressed in the paper.
When the entrance to the ear canal is occluded, e.g., by wearing an unvented hearing aid, the low-frequency component of the wearer's own voice produces a sound in the ear which is markedly different from the sound in an open ear. This so called "occlusion effect" is caused by the bone conducted low-frequency portion of the wearer's own voice which is not vented to the outside in the occluded ear canal. The uncompensated occlusion effect in a sealed ear canal manifests itself as an objectionable feeling of "echoing" in the ear and is the key reason why venting is customarily provided in hearing aids in spite of significant advantages of unvented devices.
The chief negative consequence of venting of hearing aids is that venting limits the amount of gain available before the onset of positive feedback oscillation between an outside microphone and an inside speaker. In addition, venting may reduce a desired low-frequency gain, add a vent-associated resonance, and allow background noise outside the pass band of the hearing aid to enter the ear canal and be combined with the amplified signal.
Venting may be altogether impractical for certain types of hearing aids. For instance, space limitations often preclude the use of venting with in-the-ear canal hearing aids (the most rapidly increasing style of hearing aids). As a result, one of the significant limitations sometimes created by these hearing aids is the increased sensitivity to the occlusion effect. The present invention provides an alternative to venting, whereby a significant improvement in hearing aid performance can be realized.
The Active Sound Absorber of this invention does not belong to the specie of Active Noise Reduction systems, yet a comparison may be in order. Active Noise Reduction is based on the generation of a counter-noise, i.e., a waveform precisely equal to and of opposite polarity to the noise waveform, as compared to broad band absorption provided by the Active Sound Absorber. Active Noise Reduction, e.g., as described in FIG. 2 of the U.S. Pat. No. 4,985,925 by Langberg et al is based on electroacoustical negative feedback, whereas the Active Sound Absorber typically uses positive feedback. (Please note that Langberg is also the inventor of the present invention; here Langberg et al refers to the U.S. Pat. No. 4,985,925).
The design in Langberg et al comprises a summing microphone located inside of the ear canal to provide an acoustical feedback signal, whereas in this invention no such microphone is required. Modern hearing aids are often of the in-the-ear-canal type where space is at a substantial premium and adding a summing microphone, required in an Active Noise Reduction system, to the already crowded assembly, is difficult. Further the separation of the speaker and summing microphone in an Active Noise Reduction system introduces acoustical phase shift and delay which limits performance and requires compensation to maintain stability thus complicating the design.